The proposal intends to utilize athymic, nude mice to study leprosy. 1) The cost of production of M. leprae in nude mice will be compared to that of bacilli grown in armadillos. M. leprae will be inoculated at multiple subcutaneous sites and intravenously and ultimate bacteriologic yields measured. Cost estimates of these bacilli will be compared to the cost of approximately $6.78/billion bacilli in armadillos and $9.67/billion in nude mice inoculated in a single footpad. A continuing supply of M. leprae, particularly uncontaminated, viable organisms, makes possible a variety of basic biochemical, bacteriological, phamacological, etc., studies. 2) The transmission of leprosy will be studied by administering identical inocula of viable M. leprae to nude mice by different routes (subcutaneous, intact and abraded skin, upper and lower respiratory tract, and upper and lower G.I. tract). The time course of the infection and proportional "takes" by these routes should determine the most favorable site of entry of M. leprae into new human hosts. 3) Unusual strains of M. leprae bearing different drug resistant patterns will be grown up in nude mice, cryopreserved and stored for future study.